Quantum Gravity

Quantum gravity is the missing theory that would describe gravity in the same language as everything else in physics. General relativity treats gravity as the curvature of spacetime and is superbly confirmed; quantum field theory treats the other three interactions as quantised fields on a fixed spacetime and is superbly confirmed; and where the two overlap — black-hole interiors, the first instants of the universe, the fate of information that falls through a horizon — they are mutually inconsistent. This is not a device to build. It is the physics several of the neighbouring briefs quietly rest on.

Established The obstacle usually quoted is true and misleading in the same sentence. The Planck energy is roughly fifteen orders of magnitude beyond anything a collider reaches, which is correct and frames the problem as one of energy. Two other framings have proved far more productive, and both have already produced measurements. Coherence rather than energy: hold a massive object in spatial superposition long enough for its own gravitational field to do something quantum, and the relevant scales become masses, micrometres and seconds rather than GeV. Accumulated propagation distance rather than collision energy: a Planck-suppressed effect on how a photon or a neutrino travels, integrated over gigaparsecs, becomes measurable — which is why gamma-ray observatories and a cubic kilometre of Antarctic ice have already excluded parameter space no accelerator could touch.

Frontier The framing under test on this page is that quantum gravity is approaching experimental reach, and it survives only in a much narrower form than it is usually stated in. What is approaching reach is a single question with a yes or a no at the end of it: can gravity generate entanglement between two masses? That question is sharp, the apparatus to answer it has published parameters, and it has not been built. What is not approaching reach is any experiment that would tell string theory from loop quantum gravity from causal sets from asymptotic safety, because none of those theories predicts a different answer to the question that is nearly askable.

Established The honest one-line summary of the last fifteen years: the phenomenology became an experimental science and the theory did not. This page is therefore dense with numbers, and almost every number is a limit. That is what a healthy young experimental field looks like from the inside, and it is a different thing from a field on the verge of a result.

Frontier Five theory programmes are live, none is confirmed, and each has an unsolved internal problem that its own reviewers name. Stating the programmes without stating their open problems is the standard way this subject gets oversold, so the open problems come first. String and M-theory with holography treats gravity as one excitation of an extended object, and the gauge/gravity duality gives a nonperturbative definition of quantum gravity in a spacetime with a boundary. Established That duality is a genuine achievement and it is exact in anti-de Sitter space. Speculative Our universe is not anti-de Sitter, and the swampland programme — the attempt to say which effective field theories admit a ultraviolet completion at all, through the Weak Gravity Conjecture and the de Sitter conjecture — consists of conjectures that its own review calls conjectures.

Frontier Loop quantum gravity quantises geometry itself in a background-independent way, producing discrete area and volume spectra, and is aimed squarely at the big bang and the black-hole interior where the smooth continuum is expected to fail. Its acknowledged open task, in its own short review, is the classical limit: recovering low-energy general relativity and ordinary quantum field theory from the discrete structure. Frontier Asymptotic safety proposes that gravity is renormalisable non-perturbatively through an interacting ultraviolet fixed point. Its own participants published a paper of critical reflections clarifying the programme's open questions and what they described as misunderstandings about it — insiders naming their own unfinished business, which is the form of evidence this page weights most heavily. The fixed point's existence in full, truncation-independent form is not settled.

Frontier Causal set theory makes spacetime fundamentally discrete — a locally finite partial order — and is the one discrete approach that keeps local Lorentz invariance exactly. Its open problems are the continuum limit, the dynamics and action principle, and the characteristic non-locality that discreteness plus exact Lorentz invariance forces on it. Frontier Causal dynamical triangulations is a nonperturbative lattice regularisation of the gravitational path integral with causality imposed by construction; its open questions are whether a ultraviolet fixed point exists and can be located, and what the quantum geometries in each phase actually are. Speculative These are not five attempts at the same object. Two are discrete, one is continuum, one is background-independent, one is unified-matter. A result confirming any of them would not merely rank the others; it would reclassify what kind of question “what is spacetime” is.

Established The measured record on the laboratory side is real, quantitative, and about classical gravity. Gravitational coupling has been measured between millimetre-sized bodies: 90 milligram gold spheres of about a millimetre radius at a minimum surface separation of 400 micrometres, with the time-dependent gravitational acceleration measured to a systematic accuracy of 4 × 10⁻¹¹ m/s² and a statistical precision of 4 × 10⁻¹² m/s², consistent in signal strength with a 1/r potential. Established A 0.4 milligram magnetically levitated particle in a superconducting trap, with kilogram-scale source masses about half a metre away, reached a force noise of 0.5 femtonewtons per root hertz at 26.7 Hz with a mechanical quality factor above 10⁷ and all six degrees of freedom tracked — gravity measured in the attonewton regime with a levitated sensor, which is the architecture the entanglement experiments need. Established And atom interferometry has been done by holding atoms in optical lattices rather than dropping them, giving a combined accuracy of 6.2 nm/s², four times better than the best free-fall equivalents.

Established Quantum control of massive objects has also arrived, and it is the enabling capability rather than the result. A 143 nanometre silica nanoparticle, optically levitated, has been cooled from room temperature to its motional quantum ground state — mean occupation 0.43 ± 0.03 phonons, effective temperature 12 microkelvin, seven orders of magnitude of cooling along the cavity axis. Established None of these are quantum-gravity experiments. Westphal's spheres, the levitated milligram sensor and the lattice-held atoms all measure classical Newtonian gravity with extraordinary precision. They are prerequisites, and calling them tests of quantum gravity is the single commonest overclaim about this field.

Established The astrophysical constraints are where this subject has genuinely established results, and they are all limits. Fermi's observation of a 31 GeV photon from the short hard burst GRB 090510 set limits on a linear energy dependence of the photon propagation speed that required, for the first time, a quantum-gravity mass scale significantly above the Planck mass. Established LHAASO went much further: from twelve ultra-high-energy gamma-ray sources above 100 TeV, with the highest photon at 1.4 PeV, the two highest-energy sources give a first-order Lorentz-invariance-violation scale above roughly 10⁵ times the Planck mass and a second-order scale above 10⁻³ times the Planck mass, each improving previous limits by at least an order of magnitude. Frontier MAGIC's bounds from GRB 190114C, using the most energetic photons ever seen from a gamma-ray burst at above 0.2 TeV, are competitive at quadratic order — but they are conditional on stated assumptions about the burst's intrinsic spectral and temporal behaviour, and that conditionality is why they are flagged one level weaker here than LHAASO's.

Established Two more nulls close off the most-watched channels. IceCube searched for quantum-gravity effects in the flavour composition of astrophysical neutrinos and placed what the collaboration describes as the most stringent limits of any known technology on the dimension-six operators that parameterise space-time defects, reaching 10⁻⁴² GeV⁻². Established The LIGO–Virgo–KAGRA tests of general relativity with GWTC-3, across fifteen high-confidence signals, examined post-Newtonian deformation coefficients, spin-induced quadrupole moments, gravitational-wave dispersion, non-general-relativistic polarisations, post-merger echoes, the graviton mass and ringdown quasi-normal modes. The result was no deviation from general relativity on any of them, residual power consistent with detector noise, and a graviton-mass bound of 2.42 × 10⁻²³ eV/c². Established The echo search matters more than its billing suggests: it was the most direct available probe of Planck-scale structure at a horizon, and it came back empty.

Established One gravity-related theory has actually been falsified, and it is the strongest single result in this brief. The Diósi–Penrose model proposes that spatial superposition collapses as a back-reaction from spacetime being curved differently by each branch, and it predicts spontaneous radiation emission. An experiment at the Gran Sasso underground laboratory measured that emission rate and ruled out the natural, parameter-free version of the model, setting a lower bound on the effective nuclear mass-density size about three orders of magnitude above previous bounds. Frontier The wider collapse-model literature — continuous spontaneous localisation, trace dynamics, gravity-induced collapse — is reviewed with its constraints from molecular interferometry and optomechanics, and the review's own conclusion is that laboratory tests over the following two decades could verify or eliminate these modifications. That is a falsifiable programme being falsified, which is what the process is supposed to look like.

Frontier Analogue gravity is real physics and is routinely misdescribed. In a Bose–Einstein condensate analogue black hole, spontaneous Hawking radiation has been observed with correlations between escaping particles and their trapped partners, and the entanglement verifies the quantum nature of that analogue radiation — high-energy pairs entangled, low-energy pairs not. Established A follow-up measured the correlation spectrum and found it thermal at the Hawking temperature implied by the analogue surface gravity. Established What this establishes is that the derivation works: quantum field theory on a curved background with a horizon produces thermal, entangled radiation. Handwave What it says about whether gravity is quantised, about black-hole entropy, or about the information paradox is nothing at all.

Established And the arithmetic that governs the whole page. The gap between the heaviest object ever brought to interfere with itself and the lightest mass the entanglement proposal needs is roughly ten orders of magnitude. The proposal that reduced the requirement by one to two orders of magnitude was published in 2020 and still describes an unbuilt apparatus. Established Set that beside the constraint side — a first-order Lorentz-violation scale pushed to 10⁵ Planck masses, a dimension-six limit at 10⁻⁴² GeV⁻², eight null tests of general relativity including echoes, and one dead collapse model — and the shape of the field is legible. The nulls are strengthening faster than the positive-signal experiments are maturing.

Frontier The live positions in this subject are unusually well separated, and it is worth setting them out as positions with holders rather than as a consensus with a fringe. Several are mutually exclusive; at least two have already been settled by measurement; one is settled in the sense that its parameter-free version is dead and its parametrised survivals are not.

Frontier Position one: gravity is a quantum field, the graviton exists, and a gravitationally induced entanglement signal will be seen. This is the position of the group that proposed witnessing entanglement through spin correlations on one embedded spin per mass, with Casimir–Polder and other forces excluded by construction, and of the companion information-theoretic argument for what a positive result would mean. Frontier Position two, held inside the same literature: a positive result would not strictly prove field quantisation. Whether entanglement generation strictly implies that the mediator is quantised, or admits mediators that are non-classical without being quantum, is contested in the review literature and treated there as an open interpretive question rather than a settled inference. The brief takes no side; the disagreement is a fact about the field and belongs on the page.

Established Position three: gravity causes wave-function collapse. In its parameter-free Diósi–Penrose form this is now experimentally excluded by the Gran Sasso spontaneous-radiation measurement, which is a rare and clean outcome. Speculative Parametrised survivals of the same idea remain available and remain untested at the parameters that survive, which is a much weaker position than the original and should be described as one.

Frontier Positions four to seven are the theory programmes, and the honest statement about all four is that no experiment currently distinguishes them. Spacetime is fundamentally discrete and exactly Lorentz-invariant (causal sets); spacetime is a lattice-regularised path integral with an ultraviolet fixed point (causal dynamical triangulations); gravity is asymptotically safe; geometry is quantised with discrete spectra (loop quantum gravity). Speculative Each has a community, a review literature and an internal open problem. None of them predicts a different answer to the entanglement question, which is the only question anyone is close to asking.

Established Position eight: gauge/gravity duality is the correct nonperturbative definition of quantum gravity. As a statement about anti-de Sitter space this is established as a duality. Speculative As a statement about our universe it is not, and the distinction is load-bearing rather than pedantic. Speculative Position nine: most effective field theories cannot be completed into a quantum theory of gravity at all — the swampland. The programme's own review presents the Weak Gravity Conjecture and the de Sitter conjecture as conjectures, and does not address their testability. A brief that reported them as constraints would be reporting a research programme as a result.

Frontier Position ten: Lorentz invariance is violated at the Planck scale in a way astrophysics can see. This is the position that built the multi-messenger phenomenology programme across photons, neutrinos, cosmic rays and gravitational waves, and it is a serious, productive research direction. Established The linear-order version of it is now heavily constrained — excluded a hundred thousand times beyond the scale where it was expected. Frontier A confirmed energy-dependent propagation delay near the Planck scale, surviving intrinsic-emission modelling, would select for Lorentz-violating approaches and against causal set theory, which keeps local Lorentz invariance exactly. That is the one channel where a positive result would actually discriminate between programmes, and it is the channel returning the strongest nulls.

Speculative Position eleven: black-hole horizons carry Planck-scale structure that produces echoes in the gravitational-wave ringdown. This was an attractive idea because it was directly testable with existing instruments. It was directly tested in GWTC-3 and the residual power was consistent with detector noise. Handwave Position twelve: analogue systems can settle questions about real gravity. This is the strong reading of the analogue-gravity programme and the sources themselves do not make it; they make the weaker and correct claim about the field-theoretic derivation. The strong reading is common in popular accounts and has no support in the primary literature.

Speculative And the question underneath all twelve, which nobody has an experimental handle on: is a theory of quantum gravity the kind of thing that gets confirmed? The programmes are distinguished by structure rather than by prediction, the one nearly-askable experimental question is common to all of them, and the channel that would discriminate is returning limits. It is entirely possible that the theory question is settled sociologically before it is settled empirically, and the field should be honest that this is a live possibility rather than a cynical aside.

Frontier The binding bottleneck is decoherence, and it has a number attached. The entanglement witness needs two masses each held in spatial superposition, interacting only gravitationally, for long enough that a gravitationally generated phase becomes measurable. Gravity is the weakest interaction by roughly thirty-nine orders of magnitude relative to electromagnetism, so the experiment is a race between an extraordinarily faint signal and every other force, every stray field, every gas molecule and every thermal photon. The apparatus is not hard to describe and is very hard to build.

Established The second bottleneck is the mass gap, and it is the most informative single quantity in this brief. The heaviest object ever brought to interfere with itself and the lightest mass the entanglement proposal requires are separated by about ten orders of magnitude. Every enabling result in section 2 — ground-state cooling of a nanoparticle, levitated milligram force sensing, gravity between millimetre spheres — addresses one side of that gap or the other, and none of them closes it. Frontier The Casimir-screened proposal, which interposes a conducting plate between the interferometers, lowers the mass requirement by one to two orders of magnitude and its authors say so explicitly. That is real progress on a gap of ten orders, and it should be reported as such rather than as an arrival.

Frontier The third bottleneck is a magnet. The Stern–Gerlach scheme that creates the superposition requires a field gradient of order 10⁴ tesla per metre across the region where a picogram mass with an embedded spin is held. That is a demanding but specifiable piece of hardware, and it is the kind of bottleneck that money and engineering can attack — unlike the theory bottleneck below, which they cannot.

Frontier The fourth bottleneck is on the theory side and is not an instrument problem. No candidate theory produces a unique, sharp, low-energy prediction that differs from its rivals at an accessible scale. The experimental programme and the theoretical programme are, at present, not connected: the experiments that can be done do not discriminate among theories, and the discriminating observables are at energies nobody reaches. Speculative A theorist producing a prediction that a tabletop or an observatory could falsify would change this field more than any instrument would.

Frontier And a fifth, which is about the astrophysical channel specifically. Every propagation-delay limit is conditional on modelling the intrinsic behaviour of the source — how a gamma-ray burst emits at different energies, and when. MAGIC's paper states its assumptions; LHAASO's approach relies on the highest-energy photons from steady sources rather than on burst timing, which is why its limits are flagged more strongly here. Improving these limits further is partly an astrophysics problem about sources rather than a physics problem about spacetime, and that ceiling is now visible.

Frontier This brief waits on capabilities rather than on results other briefs produce, which is why its adjudication records requirements and no dependency edge. The capabilities are specific: matter-wave interferometry at masses many orders above molecular scale, levitated optomechanics with coherence times of order a second, magnetic field gradients of order 10⁴ T/m, and vacuum and vibration isolation good enough that gravity is the only interaction left. Each is an engineering target with a number, and none of them is on this map as a brief.

Established What it does not wait on is more astrophysics. The constraint channel is productive and will keep improving, but it is improving limits on hypotheses that are already excluded at the natural scale. LHAASO at 10⁵ Planck masses and IceCube at 10⁻⁴² GeV⁻² are not one order of magnitude away from a discovery; they are a hundred thousand times past where the effect was expected. Frontier A next-generation observatory would sharpen a number that is not the binding one.

Frontier Several briefs on this map depend on this one, and the traffic runs in that direction only. Gravity Modification records a formal dependency on this brief, because every constraint in that subject is a classical-regime measurement and the only regime in which a controllable coupling could hide is the one quantum gravity governs. Warp Drives, Wormholes, Spacetime Metric Engineering and Negative Mass all inherit the same structure: their terminal question is whether the quantum stress-energy of fields in curved spacetime permits something general relativity plus classical matter forbids. Handwave None of them supplies anything to this brief in return, and a page that implied otherwise would be inventing an edge.

Established The instrument dependency is shared with a field that has nothing to do with gravity. Levitated optomechanics, ion and atom interferometry, superconducting traps and cryogenic vibration isolation are developed for quantum sensing, inertial navigation and fundamental metrology. The quantum-gravity programme is a customer of that industry rather than its driver, and that is a fair description of where the enabling hardware actually comes from.

Frontier The experiments worth ranking are ranked by what a result would settle, not by how hard they are. Four tiers, and only the third is currently producing outcomes.

Frontier Tier one, decisive-ish: a gravitationally induced entanglement signal. Two masses in adjacent interferometers, each in spatial superposition, interacting only gravitationally, with a conducting plate screening the Casimir–Polder interaction, read out through spin correlations, with independent replication. A positive result would establish that the gravitational mediator is not classical, under the assumption that entanglement cannot be created by local operations and classical communication. Frontier It would not select among string theory, loop quantum gravity, causal sets, asymptotic safety or causal dynamical triangulations, because none of them predicts a different answer. That limitation should be stated whenever the experiment is described, and usually is not.

Frontier Tier two, discriminating but conditional: a confirmed energy-dependent propagation delay. A photon or neutrino delay at a scale near the Planck mass, surviving intrinsic-emission modelling, would select for Lorentz-violating approaches and against causal set theory. Established Current status: null, with the first-order constraint pushed to 10⁵ Planck masses. This is the only channel where a positive result would sort the theory programmes, and it is the channel with the strongest exclusions.

Established Tier three, falsification of a specific model, and it is already happening. The parameter-free Diósi–Penrose model made a prediction about spontaneous radiation, an underground laboratory measured that radiation, and the model died. Frontier A separate quantum-optics route proposes probing a modified Heisenberg uncertainty relation arising from a minimal length, by examining the canonical commutator of the centre-of-mass mode of a mechanical oscillator with mass near the Planck mass — claimed by its authors to be within reach of current technology, which is a claim worth carrying and worth flagging as the proposers' own.

Speculative Tier four is not an experiment but the argument about what tier one would mean, and it needs to be resolved in parallel rather than afterwards. If the community cannot agree in advance whether an entanglement signal implies field quantisation, then a positive result arrives into an interpretive dispute rather than settling one. Frontier The useful preparatory work here is theoretical: characterising precisely which non-quantum mediators could reproduce the signal, so that the experiment is designed to exclude them rather than to be argued about.

Established And the experiments that are not tests of quantum gravity but are constantly reported as though they were. Measuring gravity between 90 milligram spheres, sensing force at 0.5 femtonewtons per root hertz with a levitated 0.4 milligram particle, and measuring gravitational acceleration to 6.2 nm/s² with lattice-held atoms are all superb classical measurements. They belong on this page as prerequisites and as a record of what the enabling technology can do, and nowhere near the word “detected”.

Frontier There is no engineering payoff here, and the requirements that can be specified are all requirements for an instrument. This is prerequisite science. The brief states that plainly rather than manufacturing an application, and the institutional requirement in section 13 exists precisely because a subject with no engineering payoff has a funding problem that a subject with one does not.

Frontier The instrument specification, from the published proposal rather than from a gesture. The Casimir-screened variant of the entanglement experiment requires masses of 10⁻¹⁶ to 10⁻¹⁵ kilograms — that is 0.1 to 1 picogram — a superposition separation of about 20 micrometres, and a Stern–Gerlach magnetic field gradient of 10⁴ tesla per metre, with a conducting plate between the two interferometers to screen the Casimir–Polder force. The paper states that these masses are one to two orders of magnitude smaller than earlier proposals required, which tells you how demanding the earlier proposals were.

Established A different mass scale, an order of magnitude count away, is already routine, and confusing the two is the field's characteristic error. The levitated sensors that measure classical gravity today work at 0.4 milligrams; the sphere experiment worked at 90 milligrams. The entanglement witness wants picograms. Those differ by nine to twelve orders of magnitude, and popular coverage routinely reports the milligram achievement as though it were progress toward the picogram requirement in the direction of getting heavier. It is not the same axis: the hard part is holding a mass in superposition, and superposition gets harder as mass rises, which is why the proposal wants the lightest mass that still gravitates measurably.

Established The supporting engineering is a list of extremes with numbers attached. Ground-state cooling of an optically levitated 143 nanometre silica particle from room temperature, at 0.43 phonons and 12 microkelvin. Mechanical quality factors above 10⁷ in a superconducting magnetic trap. Force noise at 0.5 femtonewtons per root hertz. Six-degree-of-freedom tracking of a levitated milligram mass. Frontier Each of these was a headline result on its own, and the entanglement experiment needs all of them simultaneously and then something further.

Established The adjacent fields supplying the evidence in this brief are not gravity fields, and that is the point. Levitated optomechanics and matter-wave interferometry supply the laboratory capability; particle astrophysics and neutrino astronomy supply the propagation limits; gravitational-wave astronomy supplies the strong-field tests; low-background underground physics supplied the measurement that killed a collapse model. Not one of those communities is trying to build a theory of quantum gravity, which is what makes their constraints worth more than a partisan's.

Frontier Within this map, the neighbours split cleanly into those that depend on this brief and those that share its instruments. Gravity Modification depends on it formally and says so. Warp Drives, Wormholes and Spacetime Metric Engineering all have terminal questions that this subject owns; Negative Mass and Vacuum Energy Engineering own the exotic-source half of the same problem. Black Hole Physics Applications owns the astrophysical objects; Gravitational Wave Engineering owns the radiative regime, and the GWTC-3 tests sit on the seam between the two pages.

Frontier One adjacency is worth naming because it is where the confusion lives. The entanglement that the tabletop experiment would witness is the same phenomenon that quantum information science manipulates routinely at atomic scale. What is new is not the entanglement but the mediator: everything else that entangles two systems does so through a field we already know is quantum. The whole content of the experiment is the identity of the thing in the middle.

Frontier The institutional requirement is unusual on this map because the subject has no engineering payoff to sell, and the brief's adjudication records that as a formal constraint rather than as a sentiment. A programme whose best-case output is a yes/no about the mediator of the weakest interaction, on a timescale no funding cycle matches, needs patient money and it needs an institution willing to say out loud that the money buys understanding rather than capability.

Established The evidence base for this brief was overwhelmingly produced by facilities built for other reasons, and that is the most transferable institutional observation available. IceCube exists to do neutrino astronomy. LHAASO exists to do cosmic-ray and gamma-ray astronomy. LIGO, Virgo and KAGRA exist to detect gravitational waves. The Gran Sasso laboratory exists to run low-background experiments. Every one of the established results in section 2 is a by-product. Frontier That is an efficient arrangement and a fragile one: the constraints improve when those fields are funded and not otherwise, and no agency is deciding how much quantum-gravity phenomenology to buy.

Frontier The positive-signal experiment has the opposite problem: it is nobody's by-product. A dedicated two-interferometer apparatus with picogram masses, a screening plate and a 10⁴ T/m gradient has no other use, will produce nothing publishable for years, and can fail in a way that produces no paper at all. Speculative It is exactly the kind of instrument that patient foundational funding exists to buy and that competitive short-cycle funding cannot.

Established There is a positive institutional exhibit in this record and the page should name it. A theory made a parameter-free prediction; an underground laboratory measured the relevant quantity; the theory died; the authors of the measurement included the theory's own originator among their number. That is a complete falsification cycle executed in a subject widely described as untestable, and it is a better argument for funding this field than any of the promissory notes usually offered on its behalf.

Frontier The hazards here are epistemic rather than physical, and the principal one is overclaim. “Quantum gravity in the laboratory” is a phrase attached, in popular coverage, to experiments that measured classical Newtonian gravity very precisely. The distinction between a prerequisite and a test is the whole content of section 2, and eroding it costs the field its ability to say what would count as a result.

Established The second consideration is about how the vocabulary of this subject is borrowed. “Quantum vacuum”, “spacetime engineering” and “zero-point energy” are real terms in this literature and are also the standard furniture of propulsion claims with no mechanism. This brief's residual honest position — that the quantum-gravitational regime is untested and therefore cannot be used to rule things out — is regularly repurposed as though it were a positive argument for a device. It is not. An untested regime supports no engineering claim in either direction.

Frontier Third, there is a real allocation question and it deserves better than a slogan. This is expensive, slow, prestigious work with no application, competing for the same theoretical and experimental talent as condensed matter, quantum information and precision metrology. Speculative The defensible answer is not that quantum gravity is more important but that its instruments are shared: the levitated optomechanics and interferometry it needs are the same technologies that produce quantum sensors and inertial navigation, so the programme is not a pure transfer out of applied physics.

Established Fourth, and against the field's own habits: the discipline of publishing limits is a virtue that needs institutional protection. Every established result in this brief is a null. Nulls are hard to publish, hard to fund and impossible to headline, and the reason this subject has a real evidence base is that four large collaborations published theirs prominently anyway. Whatever pressure erodes that is the main ethical risk to this field's epistemic health.

Frontier A theory of quantum gravity would settle what spacetime is, and the honest statement about what follows from that is that nobody knows. The base rate is not encouraging: general relativity is a hundred and eleven years old and its most conspicuous technological consequence is a correction term in satellite navigation. Quantum mechanics is the exception that gets cited, and it is an exception rather than a rule.

Speculative Where it would matter immediately is in closing questions rather than opening capabilities. Several briefs on this map end at the same sentence: the classical constraints are decisive and the quantum-gravitational regime is untested. A theory would replace that sentence with an answer, in one direction or the other, for Gravity Modification, Warp Drives, Wormholes and Negative Mass. Handwave That the answer would be favourable is an assertion with nothing behind it, and it is the assertion most often made on this subject's behalf.

Established There is a payoff already banked and it is not the one anyone wanted. The instruments built to constrain quantum gravity are among the most sensitive ever made, and their limits constrain particle physics and astrophysics rather than propulsion: a graviton-mass bound at 2.42 × 10⁻²³ eV/c², the strongest dimension-six operator limits from any technology, first-order Lorentz-violation scales five orders past the Planck mass. Established “No theory” has not meant “no results”.

Speculative And the subject has a durable function as an epistemic exhibit whatever happens to the physics. It is the clearest available case of a question that was declared permanently beyond experiment and then acquired an experimental programme anyway — not by reaching the Planck energy, which remains impossible, but by finding two framings in which the difficulty was somewhere else. That reframing move is the transferable lesson, and it is worth more to other fields than any particular limit in section 2.

These horizons track instrument sensitivity and constraint strength, because there is no engineering programme here to track:

• 10 yr: Frontier Expect the first serious attempts at the entanglement experiment, at masses still short of the proposal's requirement, reporting on decoherence budgets rather than on signals. Established Expect the constraint side to keep improving: sharper first- and second-order Lorentz-violation limits from ultra-high-energy gamma rays, more neutrino flavour statistics, and further null tests of general relativity as the gravitational-wave catalogue grows. Speculative A positive entanglement result in this window would be a genuine surprise, and the honest base rate for it is low.
• 25 yr: Frontier This is the window in which the entanglement question plausibly gets an answer, if the picogram-scale apparatus with a 20 micrometre separation and a 10⁴ T/m gradient is actually built and replicated. Speculative A yes would establish that the mediator is non-classical and would immediately open the interpretive dispute described in section 3 rather than closing it. Speculative A no — a well-characterised apparatus at the stated parameters seeing nothing — would be at least as interesting and is the outcome nobody plans for.
• 50 yr: Speculative On the theory side, the plausible developments are consolidation rather than confirmation: programmes merging, one or two being abandoned, and the classical-limit and fixed-point problems either being solved or being recognised as structural. Handwave A completed, accepted theory of quantum gravity at this horizon is possible and unforecastable; no source in this brief's evidence base offers a timeline for one, and none can.
• 100 / 250+ yr: Handwave Beyond useful forecasting. The defensible statement is about the shape of the problem rather than a date: the discriminating observables sit at energies no conceivable instrument reaches, so a confirmed theory would have to be selected by internal consistency plus one or two indirect signatures. Speculative A field whose experimental programme in its first fifteen years produced eight strong limits and one falsification has established that it is a science; it has not established that it converges.

• Depends on No typed dependency edge, and the absence is the finding. This brief does not wait on a result another brief on this map produces — it waits on an instrument and on a theoretical idea, neither of which is a brief. The instrument is specified in section 7 and its requirement is recorded below as an industrial constraint. The theoretical idea — a candidate theory producing a unique, sharp prediction that differs from its rivals at an accessible scale — is not something any programme can be scheduled to deliver, and recording it as an edge would state a wish as a dependency.
• Requires (not on this map) Two conditions, one industrial and one institutional, and the first needs its numbers stated precisely because the token's wording and the literature's requirement are not the same quantity. The instrument. The Casimir-screened entanglement proposal asks for masses of 10⁻¹⁶ to 10⁻¹⁵ kilograms — 0.1 to 1 picogram — held in spatial superposition at a separation of about 20 micrometres, with a Stern–Gerlach field gradient of 10⁴ tesla per metre and a conducting plate screening the Casimir–Polder force. The token above says milligram, and milligrams are the scale of a different and already-working instrument: the levitated sensors that measure classical gravity today operate at 0.4 milligrams, and the sphere experiment at 90. Those two mass scales differ by nine to twelve orders of magnitude, and conflating them is the commonest error in popular coverage of this field. The token is left unchanged because it is the adjudicated string; the figures are given here because they are what the published proposal actually asks for, and a reader is better served by a page that is correct than by one that quietly repeats a token. The institution. This is prerequisite science with no engineering payoff, so it needs funding with a horizon longer than any programme cycle, tolerance for an apparatus that may produce no publishable result for years, and discipline against premature theory-of-everything claims — a discipline the field's own record on publishing nulls suggests it currently has.
• Enables The enabling reach is large and is almost entirely about closing questions rather than opening capabilities. Gravity Modification records a formal dependency on this brief, because every constraint in that subject is a classical-regime measurement and the untested quantum-gravitational regime is the only place a controllable coupling could still hide. Warp Drives, Wormholes, Spacetime Metric Engineering and Negative Mass all end at the same terminal question about the quantum stress-energy of fields in curved spacetime. No typed enabling edge is claimed to any of them beyond the one they claim to this brief, because a theory that does not exist enables nothing on a schedule.
• Adjacent Levitated optomechanics, matter-wave interferometry, atom interferometry, cryogenic and magnetic trapping and precision metrology supply the laboratory capability; particle astrophysics, neutrino astronomy, gamma-ray astronomy and gravitational-wave astronomy supply every established limit in section 2; low-background underground physics supplied the measurement that killed the parameter-free collapse model. Within this map: Gravitational Wave Engineering for the radiative regime, Black Hole Physics Applications for the objects, Vacuum Energy Engineering for the exotic-source side, and the entanglement module for the phenomenon the tabletop experiment would witness.

Established “Quantum gravity is untestable because the Planck energy is unreachable.” The Planck energy is unreachable and the conclusion does not follow. Two framings route around it: coherence rather than energy, which is the tabletop programme, and accumulated propagation distance rather than collision energy, which is why a 1.4 PeV photon and a flux of astrophysical neutrinos have already set the strongest limits anyone has. Established The subject acquired an experimental programme without anyone getting closer to the Planck energy, and that is the single most interesting structural fact about it.

Established “Quantum gravity has been tested in the laboratory.” It has not. Every laboratory result in section 2 — gravity between 90 milligram spheres, force sensing with a 0.4 milligram levitated mass, gravimetry with lattice-held atoms at 6.2 nm/s² — is a measurement of classical Newtonian gravity at extraordinary precision. Frontier They are the enabling technology for the experiment that would test quantum gravity, and that experiment has not been performed.

Established “Milligram masses in superposition — we are nearly there.” This conflates two mass scales that differ by nine to twelve orders of magnitude and moves in the wrong direction. The entanglement witness wants picograms, because superposition gets harder as mass rises and the proposal takes the lightest mass that still gravitates measurably. The milligram achievements are levitated sensors, not superpositions. Established The gap that actually matters is between the heaviest object ever interfered and the lightest proposed mass, and it is about ten orders of magnitude.

Frontier “A positive entanglement result would prove the graviton exists.” It would establish that the mediator is not a classical field, under the assumption that entanglement cannot be created by local operations and classical communication. Frontier Whether that strictly implies field quantisation, or leaves room for mediators that are non-classical without being quantum, is contested inside the review literature itself and is treated there as an open interpretive question. Anyone who tells you the experiment settles the graviton is skipping a live argument.

Handwave “Analogue black holes have shown us Hawking radiation, so we understand quantum gravity.” Analogue Hawking radiation has been observed in a Bose–Einstein condensate, its entanglement verified, and its spectrum found thermal at the analogue Hawking temperature. Established What that confirms is a derivation — quantum field theory on a curved background with a horizon — and nothing about whether gravity is quantised, about black-hole entropy, or about the information paradox. The primary sources make the weaker, correct claim; the strong reading is a popular addition.

Speculative “AdS/CFT is a theory of quantum gravity.” It is an exact duality in anti-de Sitter space, and it is a genuine nonperturbative definition of quantum gravity there. Our universe is not anti-de Sitter. Speculative “The swampland conjectures constrain physics.” The programme's own review calls them conjectures and does not address their testability. Both of these are cases where a real result gets promoted one category by omitting a qualifier.

Established “Lorentz violation has been seen.” Every result cited on this page is a limit. Fermi required a quantum-gravity mass scale significantly above the Planck mass from a single 31 GeV photon; LHAASO pushed the first-order scale past 10⁵ Planck masses; IceCube reached 10⁻⁴² GeV⁻² on dimension-six operators; GWTC-3 found no deviation on any of eight tests. Established Not one detection, and the most natural quantum-gravity signature is now excluded a hundred thousand times beyond the scale where it was expected.

Established “Gravity-induced collapse explains the measurement problem.” The parameter-free Diósi–Penrose version predicted spontaneous radiation, an underground experiment measured it, and the model was ruled out, with a lower bound on the effective nuclear mass-density size three orders of magnitude above previous limits. Speculative Parametrised versions survive at parameters that have not been probed, which is a real but much weaker position and should not be reported as the original claim.

Handwave “A theory of everything is N years away.” No source in this brief's evidence base offers a timeline for a completed theory, and the structure of the problem is that the discriminating observables sit at inaccessible energies. Speculative Any specific date is an assertion. The defensible forecast is about the experiments: the entanglement question plausibly gets an answer in twenty-five years, and answering it would not rank the theories.

Frontier And one correction this page makes to its own predecessor. An earlier version of this brief cited a 2025 Reviews of Modern Physics article by Marletto and Vedral that the research for this rewrite could not locate. It is not cited here. Established The two authors' 2017 companion argument is real, is cited below, and is a different document; the overview of gravity-mediated-entanglement proposals used in its place is also cited below. A citation that cannot be found is removed rather than repeated, and saying so is cheaper than leaving a reader to discover it.